An ink jet recording process is a process comprising ejecting ink droplets from fine nozzles of a head to form letters or figures on the surface of a recording medium such as paper. As the ink jet recording process, there has been put to practical use a process in which electric signals are converted to mechanical signals with an electrostrictive element, thereby intermittently ejecting ink droplets stored in a nozzle head section to record letters or symbols on the surface of a recording medium, a process in which part of an ink solution is rapidly heated at a portion close to an ejection portion of a nozzle head to generate bubbles, and ink droplets are intermittently ejected by volume expansion due to the bubbles to record letters or symbols on the surface of a recording medium, or the like.
As an ink jet recording ink, there has recently been provided an aqueous pigment ink in which a pigment is dispersed in water. This is because an ink using a pigment is characterized by excellent water resistance and light resistance, compared to an ink using a water-soluble dye. In such an aqueous pigment ink, it has been generally carried out that the pigment is dispersed in an aqueous dispersing medium with a dispersant such as a surfactant or a polymer dispersant.
For example, in patent document 1, it has been studied to use a polymer dispersant as the dispersant for pigment particles, and water, a non-volatile organic solvent and a lower alcohol as the aqueous medium, in a pigment ink using an acetylene glycol-based penetrant, thereby securing dispersion stability thereof. However, when the dispersant is used for dispersion of the pigment particles as described above, elements in ink preparation are increased, which makes it difficult to set ink physical properties such as viscosity to desired values. Further, also in this pigment ink, the problem that print density is difficult to be secured is not solved yet.
Furthermore, in the aqueous pigment ink, the dispersant is merely adsorbed on surfaces of the pigment particles. Accordingly, when the ink solution is ejected through fine nozzles of a nozzle head, strong shear force is applied thereto, so that the dispersant adsorbed on the surfaces of the pigment particles is eliminated to deteriorate dispersibility, resulting in recognition of the tendency of ejection to become unstable in some cases. Moreover, when the above-mentioned aqueous pigment ink is stored for a long period of time, the tendency of ejection to become unstable is also recognized in some cases.
As another technique for dispersing pigment particles in water, there has been proposed a technique of introducing sulfonic groups onto surfaces of the pigment particles. For example, patent document 2 describes a pigment ink which contains a surface-sulfonated organic pigment obtained by treating with a sulfonating agent an organic pigment dispersed in an active proton-free solvent (related art 1). According to related art 1, it has been said that the above-mentioned pigment ink has excellent dispersion stability, and good ejection stability from the nozzles of the recording head (the characteristic of being stably ejected from the recording head to a definite direction).
Patent document 3 describes that an organic pigment mass whose surface is positively chargeable is prepared by treating a sulfonic group-introduced organic pigment mass with a monovalent metallic ion, and further describes an aqueous ink composition excellent in storage stability (dispersion stability) containing fine pigment particles prepared from the organic pigment mass whose surface is positively chargeable, a dispersant and water (related art 2).
However, although an ink using the surface-treated pigment particles of the above-mentioned related art 1 or related art 2 as a colorant is excellent in dispersion stability and ejection stability, compared to conventional pigment-based ink jet recording inks, abrasion resistance of recorded matter obtained by printing on a recording medium such as plain paper or an ink jet recording medium (a recording medium having provided on a its surface an ink receiving layer for receiving an ink jet recording ink) has still been insufficient. This is considered to be due to insufficient fixability of the above-mentioned surface-treated pigment particles to the recording medium.
On the other hand, in order to improve fixability of a pigment contained in a pigment-based ink jet recording ink to a recording medium, a technique using a microencapsulated pigment in which colorant particles are coated with a polymer has been known.
Patent documents 4 and 5 propose encapsulated fine pigment particles, and patent documents 6 to 9 propose pigment particles on surfaces of which a polymer is graft polymerized. Patent document 10 proposes a method for microencapsulating a hydrophobic powder with an amphiphilic graft polymer. However, the use of a previously polymerized polymer in microencapsulating has raised the problem that the particle size after encapsulation becomes too large.
In addition to the above-mentioned proposals, patent documents 11 to 19 propose inks containing pigments coated with resins having film forming properties at room temperature by phase inversion emulsification, and patent documents 20 to 29 propose inks using pigments coated with anionic group-containing organic polymer compounds by acid precipitation.
Further, patent documents 30 to 35 propose inks using polymer emulsions in which fine polymer particles are impregnated with colorants by phase inversion emulsification (related art 3). However, in the colorant obtained by phase inversion emulsification or acid precipitation, the polymer adsorbed on the pigment particles is also sometimes eliminated and dissolved in the ink, depending on the kind of organic solvent such as a penetrant used in the ink, so that dispersion stability and ejection stability of the ink, image quality and the like have been insufficient in some cases. In the ink of related art 3, the polymer adsorbed on the pigment particles is not a little eliminated, so that the pigment content in the ink is limited from the point of dispersion stability. Accordingly, images of recorded matter obtained by using this ink have low print density. In particular, when plain paper is used as the recording medium, there have been the problems that blurring is liable to occur in the images, and that color developability is also low.
Patent Document 1: JP 3-157464 A
Patent Document 2: JP 10-110129 A
Patent Document 3: JP 11-49974 A
Patent Document 4: JP 7-94634 B
Patent Document 5: JP 8-59715 A
Patent Document 6: JP 5-339516 A
Patent Document 7: JP 8-302227 A
Patent Document 8: JP 8-302228 A
Patent Document 9: JP 8-81647 A
Patent Document 10: JP 5-320276 A
Patent Document 11: JP 8-218015 A
Patent Document 12: JP 8-295837 A
Patent Document 13: JP 9-3376 A
Patent Document 14: JP 8-183920 A
Patent Document 15: JP 10-46075 A
Patent Document 16: JP 10-292143 A
Patent Document 17: JP 11-80633 A
Patent Document 18: JP 11-349870 A
Patent Document 19: JP 2000-7961 A
Patent Document 20: JP 9-31360 A
Patent Document 21: JP 9-217019 A
Patent Document 22: JP 9-316353 A
Patent Document 23: JP 9-104834 A
Patent Document 24: JP 9-151342 A
Patent Document 25: JP 10-140065 A
Patent Document 26: JP 11-152424 A
Patent Document 27: JP 11-166145 A
Patent Document 28: JP 11-199783 A
Patent Document 29: JP 11-209672 A
Patent Document 30: JP 9-286939 A
Patent Document 31: JP 2000-44852 A
Patent Document 32: JP 2000-53897 A
Patent Document 33: JP 2000-53898 A
Patent Document 34: JP 2000-53899 A
Patent Document 35: JP 2000-53900 A